Base-catalyzed oxyalkylation has been used to prepare polyoxyalkylene polyols for many years. In base catalyzed oxyalkylation, a suitably hydric low molecular weight starter molecule such as propylene glycol or glycerine is oxyalkylated with alkylene oxide, for example ethylene oxide or propylene oxide, to form a polyoxyalkylene polyether polyol product. Because it is possible to employ a low molecular weight starter, the build ratio (polyol weight/starter weight) is relatively high, and thus the process effectively utilizes reactor capacity. Strongly basic catalysts such as sodium hydroxide or potassium hydroxide are used in these base-catalyzed oxyalkylations.
However, the bulk of polyoxyalkylene polyols useful in synthesis of polyurethane polymers as well as those suitable for other uses, contain substantial amounts of oxypropylene moieties. During base-catalyzed oxypropylation, a competing rearrangement of propylene oxide to allyl alcohol generates monofunctional species which also become oxyalkylated, producing a wide range of polyoxyalkylene monols with molecular weights ranging from that of allyl alcohol itself or its low molecular weight oxyalkylated oligomers to polyether monols of very high molecular weight. In addition to broadening the molecular weight distribution of the product, the continuous generation of monols lowers the product functionality. For example, a polyoxypropylene diol or triol of 2000 Da equivalent weight may contain from 30 to 40 mol percent monol. The monol content lowers the functionality of the polyoxypropylene diols produced from their "nominal," or "theoretical" functionality of 2.0 to "actual" functionalities in the range of 1.6 to 1.7. In the case of triols, the functionality may range from 2.2 to 2.4. As the oxypropylation proceeds further, the functionality continues to decrease, and the molecular weight growth rate slows. For these reasons, the upper practical limit for base-catalyzed polyoxypropylene polyol equivalent weight is just above 2000 Da. Even at these modest equivalent weights, the products are characterized by low actual functionality and broad molecular weight distribution.
The monol content of polyoxyalkylene polyols is generally determined by measuring the unsaturation, for example by ASTM D-2849-69, "Testing of Urethane Foam Polyol Raw Materials", as each monol molecule contains allylic termination. Levels of unsaturation of about 0.060 meq/g to in excess of 0.10 meq/g for based-catalyzed polyols such as those just described are generally obtained. Numerous attempts have been made to lower unsaturation, and hence monol content, but few have been successful.
In the early 1960's, double metal cyanide complexes such as the non-stoichiometric glyme complexes of zinc hexacyanocobaltate were found to be able to prepare polyoxypropylene polyols with low monol contents, as reflected by unsaturation in the range of 0.018 to 0.020 meq/g, a considerable improvement over the monol content obtainable by base catalysis. However, the catalyst activity, coupled with catalyst cost and the difficulty of removing catalyst residues from the polyol product, prevented commercialization. In the 1980's, interest in such catalysts resurfaced, and improved catalysts with higher activity coupled with improved methods of catalyst removal allowed commercialization for a short time. The polyols also exhibited somewhat lower monol content, as reflected by unsaturations in the range of 0.015 to 0.018 meq/g. However, the economics of the process were marginal, and in many cases, improvements expected in polymer products due to higher functionality and higher polyol molecular weight did not materialize.
Recently, researchers at the ARCO Chemical Company developed double metal cyanide complex catalysts ("DMC" catalysts) with far greater activity than ever before.
These catalysts, as disclosed in U.S. Pat. Nos. 5,470,813 and 5,482,908, incorporated herein, have again allowed commercialization under the tradename ACCLAIM.TM. polyether polyols. However, unlike the low unsaturation (0.015-0.018 meq/g) polyols prepared by prior DMC catalysts, the new, ultra-low unsaturation polyols often demonstrate dramatic improvements in polymer properties, although formulations are often different from the formulations useful with conventional polyols. These polyols typically have unsaturation in the range of 0.002 to 0.008 meq/g.
One of the drawbacks of DMC catalyzed oxyalkylation is the difficulty of using low molecular weight starters in polyether synthesis. Polyoxyalkylation of low molecular weight starters is generally sluggish, and often accompanied by catalyst deactivation. Thus, rather than employing low molecular weight starter molecules directly, oligomeric starters are prepared in a separate process by base catalyzed oxypropylation of a low molecular weight starter to equivalent weights in the range of 200 Da to 700 Da or higher. Further oxyalkylation to the target molecular weight takes place in the presence of DMC catalysts However, strong bases deactivate DMC catalysts. Thus, the basic catalyst used in oligomeric starter preparation must be removed by methods such as neutralization, adsorption, ion exchange, and the like. Several such methods require prolonged filtration of viscous polyol. The additional steps associated with catalyst removal from the oligomeric starter add significant process time and cost to the overall process. Furthermore, the higher molecular weight of the starter lowers the build ratio of the process significantly, thus decreasing reactor utilization.
A further observation connected with oxyalkylation with DMC catalysts is that a very high molecular weight component is generally observed. The bulk of DMC catalyzed polyol product molecules are contained in a relatively narrow molecular weight band, and thus DMC-catalyzed polyols exhibit very low polydispersities, generally 1.20 or less. However, it has recently been discovered that a very small fraction of molecules, i.e. less than 1000 ppm, have molecular weights in excess of 100,000 Da. This very small but very high molecular weight fraction is thought to be responsible for some of the anomalous properties observed with ultra-low unsaturation, high functionality polyols. These ultra high molecular weight molecules do not significantly alter the polydispersity, however, due to the extremely small amounts present.
In copending U.S. patent application Ser. Nos. 08/597,781 and 08/683,356, herein incorporated by reference, it is disclosed that the high molecular weight "tail" in polyoxypropylene polyols may be minimized by continuous addition of starter during oxyalkylation. In batch and semi-batch processes, low molecular weight starter, e.g., propylene glycol or dipropylene glycol, is added continuously as the polyoxyalkylation proceeds rather than all being added at the onset. The continued presence of low molecular weight species has been found to lower the amount of high molecular weight tail produced, while also increasing the build ratio, since a large proportion of the final polyol product is derived from low molecular weight starter itself. Surprisingly, the polydispersity remains low, contrary to an expected large broadening of molecular weight distribution. In the continuous addition process, continuous addition of starter during continuous rather than batch production was found to also result in less low molecular weight tail, while allowing a build ratio which approaches that formerly obtainable only by traditional semi-batch processing employing base catalysis.
Unfortunately, it has been observed that when glycerine, a widely used trifunctional starter, is employed in either the batch-type continuous addition of starter process, or the continuous-type continuous addition of starter process, the DMC catalyst gradually deactivates, and often a polyether of the desired molecular weight cannot be obtained, or when obtained, product characteristics such as amount of high molecular weight tail, polydispersity, etc., are less than optimal. This has been found to be the case even when the glycerine addition is relatively slow, but is exacerbated when the glycerine addition rate is increased, as may happen during commercial production by normal or abnormal process excursions, pump failure, and the like.
It would be desirable to be able to utilize low molecular weight starter molecules for polyol production using DMC catalysis. It would further be desirable to prepare DMC-catalyzed polyols with minimal high molecular weight tail components. It would be further desirable to prepare polyoxyalkylation polyols in high build ratios. However, these objectives cannot be met if catalyst deactivation occurs.